Production method of belt for stainless steel continuously variable transmission belt

ABSTRACT

When a metastable austenitic stainless steel strip with a value Md(N), which is calculated from a composition, of 20–100 is ring-rolled to a steel belt, the relationship of −0.3913T+0.5650Md(N)+60.46ε≧65.87 is established among a material temperature T, an equivalent strain ε and the value Md(N). Due to the controlled rolling, a stainless steel belt for a continuously variable transmission is bestowed with fatigue properties similar or superior to those of a 18%-Ni maraging steel belt. The value Md(N) is defined by the equation of Md(N)=580−520C−2Si−16Mn−16Cr−23Ni−300N−10Mo, and the equivalent strain ε is defined by the equation of ε=√{square root over (4(1n(1−R)) 2 /3)} (R: reduction). Furthermore, the steel belt is stabilized in its quality and profile by confining a variation ΔT of the material temperature T within a range of ±6.4° C.

INDUSTRIAL FIELD

The present invention relates to a ring-rolling method of manufacturinga continuously variable transmission belt from a metastable austeniticstainless steel strip.

BACKGROUND OF THE INVENTION

Such a material with high strength as 18% Ni-maraging steel has beenused so far for a continuously variable transmission belt. A metastableaustenitic stainless steel is sometimes used for the purpose, asdisclosed in JP 2000-63998A. The continuously variable transmission beltis conventionally manufactured by the following steps: A steel strip isformed to a ring shape by plasma- or laser-welding its front and tailends together. The welded steel strip is heat-treated to eliminate ahardness difference between base and welded parts and smoothened at itsedge by barreling. The steel strip is then ring-rolled to apredetermined thickness and stretched to a predetermined circumferentiallength. Thereafter, the steel strip is nitrided and aged so as to hardenits surface layer.

The manufactured steel belt is subjected to a rotation-tensile fatiguetest or the like for evaluation of fatigue properties. 18% Ni-maragingsteel, which is strengthened by work-hardening and aging (strain-aging),has excellent fatigue properties due to a hard nitrided surface layerand effects of cold-working on mechanical properties. However, 18%Ni-maraging steel is scarcely work-hardened due to its large deformationresistance, so as not to anticipate an increase of strength derived fromwork-hardening even by ring-rolling with a heavy duty. The heavy-dutyrolling often causes damages of a steel strip during rolling, when thesteel strip lacks of ductility.

A metastable austenitic stainless steel is also a kind of steel, whichis work-hardened or strain-aged by cold-rolling. Its strength isremarkably improved by formation of strain-induced martensite andwork-hardening of residual austenite in comparison with 18% Ni-maragingsteel, but its strengthening rate is varied in correspondence to amaterial temperature during rolling. Heat generation and dissipationduring rolling put significant effects on mechanical properties of arolled steel strip or belt. In this consequence, a steel beltmanufactured by ring-rolling has thickness, width and cross-sectionalhardness deviated in response to a manufacturing season.

In short, it is difficult to manufacture a steel belt, which has stablematerial strength necessary for use as a continuously variabletransmission belt. The difficulty is somewhat caused by mechanicalproperties of the metastable austenitic stainless steel.

SUMMARY OF THE INVENTION

An object of the present invention is to manufacture a steel belt, whichhas stable properties necessary for a continuously variabletransmission, from a metastable austenitic stainless steel strip byring-rolling the steel strip under properly controlled conditions.

According to the present invention, a metastable austenitic stainlesssteel strip is used as a material of a continuously variabletransmission belt. The metastable austenitic stainless steel strippreferably has a value Md(N) controlled within a range of 20–100,wherein the value Md(N) is determined by a chemical composition of thesteel according to the formula of:Md(N)=580−520C−2Si−16Mn−16Cr−23Ni−300N−10Mo.

After the steel strip is formed to a ring shape by welding its front andtail ends together, it is ring-rolled under the condition that arelationship of −0.3913T+0.5650Md(N)+60.46ε≧65.87 is established among amaterial temperature T (° C.), an equivalent strain ε and the valueMd(N). The equivalent strain ε is represented by the formula ofε=√{square root over (4(1n(1−R))²/3)}, wherein R is a reduction ratio. Atemperature of a rolling atmosphere or a surface temperature of thesteel strip at a position just before a work roll may be used as thematerial temperature T. Furthermore, when the steel strip is ring-rolledunder the condition that a fluctuation ΔT(° C.) of the materialtemperature T is confined within a range of ±6.4° C., a rate ofstrain-induced martensite is controlled to a predetermined value with atolerance of 5 vol. %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a ring-rolling mill.

FIG. 2 is a block diagram for explaining a temperature control system.

FIG. 3 is a graph showing effects of a value Md(N) and a rollingtemperature on formation of strain-induced martensite.

FIG. 4 is a graph showing an effect of a material temperature onformation of strain-induced martensite.

FIG. 5 is a schematic view illustrating a bending-stretching fatiguetesting machine for measuring fatigue properties.

FIG. 6 is a graph showing fatigue properties of a continuously variabletransmission belt made of a metastable austenitic stainless steel, whichis strengthened by ring-rolling, in comparison with another continuouslyvariable transmission belt made of 18% Ni-maraging steel.

FIG. 7 is a graph showing a rate of strain-induced martensite inrelation with a material temperature.

FIG. 8 is a graph showing distribution of cross sectional hardness alonga distance from a welding point.

FIG. 9 is a view showing points for measuring cross-sectional hardnessin the vicinity of a welding point.

PREFERRED EMBODIMENTS OF THE INVENTION

When a metastable austenitic stainless steel strip is cold rolled, it isstrengthened by formation of strain-induced martensite andwork-hardening of residual austenite. A rate of strain-inducedmartensite is varied in response to a temperature and a reduction ratioR during cold-rolling as well as a value Md(N). For instance, formationof strain-induced martensite is intensified as falling of the rollingtemperature with the provision that the value Md(N) and the reduction Rare constant, resulting in improvement of strength. An increase of thestrain-induced martensite also leads to upgrading of cross-sectionalhardness.

Dependency of material strength on a rate of strain-induced martensiteis advantageously used as a parameter for imparting a predeterminedfatigue strength to a steel belt. If a rate of strain-inducedmartensite, which is formed by ring-rolling, necessary for a certainfatigue strength is known beforehand, such rolling conditions as amaterial temperature T, an equivalent strain ε and a reduction ratio Rcan be preset in order to gain the forecast rate of strain-inducedmartensite.

The inventors have searched and examined effects of compositions,rolling temperatures and strains on a rate of strain-induced martensitefor provision of a metastable austenitic stainless steel strip withfatigue strength similar or superior to 18% Ni-maraging steel, anddiscovered the ring-rolling conditions that properties suitable for acontinuously variable transmission belt are imparted to a rolled steelstrip without necessity of aging treatment or by moderate aging. Thatis, when a steel strip is ring-rolled under the condition that arelationship of −0.3913T+0.5650Md(N)+60.46ε≧65.87 is established among amaterial temperature T (° C.), an equivalent strain ε and a value Md(N),strain-induced martensite is formed at a rate necessary for apredetermined fatigue strength. Furthermore, the rate of strain-inducedmartensite is controlled with a deviation of 5 vol. % by confining afluctuation ΔT of the material temperature T within a range of ±6.4° C.during ring-rolling

A metastable austenitic stainless steel suitable for the purposepreferably has a value Md(N) within a range of 20–100.

If the value Md(N) is less than 20, strain-induced martensite is notformed at a rate enough to enhance strength, unless a steel strip isring-rolled or cold-worked at an extremely low temperature withindustrial difficulty. The low value Md(N) does not assureaustenite/martensite transformation for improvement of fatigue strength,on use of the steel strip as a continuously variable transmission belt.Moreover, an austenite phase is more stable as a decrease of the valueMd(N), so that a rate of strain-induced martensite does not reach 80vol. % or more at a surface layer of the steel strip and that it is alsodifficult to form strain-induced martensite at a rate of 60 vol. % ormore with high reliability. As a result, surface nitriding reaction doesnot progress to an extent necessary for improvement of wear-resistanceand fatigue strength. On the other hand, a steel strip, which has acomposition with a value Md(N) above 100, is transformed to martensiteat a too early stage due to deformation on its use as a continuouslyvariable transmission belt, so that fatigue strength is rather lowered.

After a steel strip is formed to a ring shape, it is ring-rolled by arolling mill, as shown in FIG. 1. The steel strip 1 is ring-rolled by acouple of work rolls 2 a, 2 b during traveling between a tension roll 3and a return roll 4. A 4-high rolling mill, which has back-up rolls forsupporting the work rolls, may be also employed. Such rolling conditionsas rolling load, tension and circumferential speed of work rolls areproperly determined as follows:

The steel strip 1 is sent to a gap between the work rolls 2 a and 2 band gradually reduced in thickness during traveling along an endlesstrack. During rolling, expansion of the steel strip 1 along itscircumferential direction is compensated by elongation of a distancebetween axes of the rolls 3 and 4 in order to keep a tension, which isapplied to the steel strip 1, at a constant value. Loads, which are puton the rolls 2 a, 2 b, 3 and 4, are controlled by a load cell 5. Thecircumferential length of the steel strip 1 is calculated from diametersof the rolls 3, 4 and the distance between the axes of the rolls 3 and 4measured by a range finder 6.

A material temperature T is kept at a value within a predetermined rangeby a temperature control system, as shown in FIG. 2. In the temperaturecontrol system, a temperature of the steel strip 1 is measured by anoncontact radiation thermometer 9 at a position where the steel strip 1is just sent to the gap between the work rolls 2 a and 2 b. The measuredvalue is outputted to a digital-indicating controller 7. A volume of hotair, which is fed from a generator 8 to a heating box 10, and a volumeof waste air, which is returned from the heating box 10 to the generator8, are controlled by commands from the controller 7, so as to keep thesteel strip 1 at a temperature within a predetermined range. Of course,the material temperature T can be kept within the predetermined range bycontrolling a rolling atmosphere, instead of the temperature controlsystem shown in FIG. 2.

When the steel strip 1 is ring-rolled under the conditions that thevalue Md(N) and the reduction R are held constant, a rate ofstrain-induced martensite to a metallurgical structure of a manufacturedsteel belt becomes bigger as the material temperature T falls down, asshown in FIG. 3. Cross-sectional hardness of the steel belt becomeshigher as an increase of strain-induced martensite α′. Formation ofstrain-induced martensite α′ is also accelerated by increase ofreduction R or a value Md(N), even when the steel strip 1 is rolled at aconstant material temperature T.

These effects of the material temperature T, the value Md(N) and thereduction R on formation of strain-induced martensite indicate that arate of strain-induced martensite in a manufactured steel belt isadjusted to a certain value by interactions of the material temperatureT, the value Md(N) and the reduction R. The inventors have arranged therelationship of FIG. 3, which shows the effects of the materialtemperature T, the value Md(N) and the reduction R on a rate ofstrain-induced martensite α′, by multiple regression analysis anddiscovered that a relationship ofα′=−0.3913T+0.5650Md(N)+60.46ε−10.87is established among the rate of strain-induced martensite α′, thematerial temperature T, the value Md(N) and an equivalent strain ε,wherein the equivalent strain ε is represented by ε=√{square root over(4(1n(1−R))²/3)} in relation with the reduction R.

By the way, a steel belt, which is manufactured by ring-rolling a steelstrip at a material temperature T of 0° C., 25° C. or 50° C. with aconstant value Md(N) and a constant reduction ratio R, has themetallurgical structure that a rate of strain-induced martensite α′ isvaried in relation with the material temperature T, as shown in FIG. 4.Variation of strain-induced martensite α′ also puts effects on fatigueproperties of the steel belt.

In fact, a fatigue test was performed, using a bending-stretchingfatigue testing machine, wherein a test piece 12 was fixed to asubsidiary belt 13 with a snap pin 11 and disposed between a drivingpulley 14 of 70 mm in diameter and a testing pulley 15 with a diameter D(mm), as shown in FIG. 5. The driving pulley 14 was rotated at 500r.p.m., while a constant tension F (39.2 N/mm²) was applied to the testpiece 12.

Under these conditions, a maximum stress σ_(max) is calculated accordingto the formula of σ_(max)=T+E·t/2ρ, wherein E is Young's modulus, t isthickness (mm) of the test piece 12 and ρ is a bend radius [ρ=(D+t)/2].Calculation results in FIG. 6 prove that a fatigue strength, which issubstantially the same as a conventional 18%-Ni maraging steel belt, isgained by a rate of strain-induced martensite α′ not less than 55 vol. %at a material temperature T of 25° C. or lower. By substitution of α′≧55vol. %, the above-mentioned formula is rewritten to:−0.3913T+0.5650Md(N)+60.46ε≧65.87

The rate of strain-induced martensite α′ is also variable in relationwith an atmospheric temperature during ring-rolling. For instance,dissipation of processing heat is varied in correspondence to anatmospheric temperature different between winter and summer seasons.Variation of the heat dissipation leads to seasonal fluctuations in arate of strain-induced martensite α′, even when a metastable austeniticstainless steel strip is ring-rolled under the same conditions.Fluctuations in the rate of strain-induced martensite α′ cause change ofdeformation-resistance of the steel strip 1, and finally inducedeviations of thickness, width and hardness in a manufactured steelbelt.

Parameters, i.e. the value Md(N) and the equivalent strain ε, in theformula of α′=−0.3913T+0.5650Md(N)+60.46ε−10.87 can be regarded asconstants, which are determined by a reduction ratio R calculated froman original thickness of a steel strip 1 and a target thickness of amanufactured steel belt. The remaining parameter, the materialtemperature T, is variance, which is influenced by heat generation andheat dissipation during ring-rolling as well as seasonal change of anatmospheric temperature. In this sense, the formula ofα′=−0.3913T+0.5650Md(N)+60.46ε−10.87 for determination of a rate ofstrain-induced martensite α′ is rewritten to the formula ofα′=−0.3913T+A+B (A and B are constants) involving the materialtemperature T as only one parameter. The constants A, B are deleted fromthe formula by handling a variation ΔT of the material temperature Tduring ring-rolling and a variation Δα′ as indices, and the formula isrewritten to Δα′=−0.3913ΔT.

Even when a material temperature T is kept at a constant value, a rateof strain-induced martensite α′ is fluctuated, as noted in FIG. 4. Thatis, a deviation of approximately 5 vol. % is noted at any materialtemperature T of 0° C., 25° C. and 50° C. A rate of strain-inducedmartensite α′, which is formed by ring-rolling under the condition thatthe material temperature T is kept at a fixed value, is fluctuated witha variation within a range of ±2.5 vol. %. By substitution of −2.5≦Δα′≦2.5, the formula of Δα′=−0.3913ΔT is rewritten to:−6.4≦ΔT≦6.4

The formula of −6.4≦ΔT≦6.4 means tolerance of the material temperature Tfor production of a steel belt with stable quality characteristics,wherein a variation Δα′ of strain-induced martensite α′ is controlledwith fluctuations within a range of 5 vol. % when a steel strip 1 isring-rolled at a constant material temperature T with a constant valueMd(N) and a constant reduction ratio R. In short, a variation Δα′ ofstrain-induced martensite α′ is confined within a range of 5 vol. % bycontrolling a material temperature T with a variation within a range of±6.4° C. during ring-rolling, resulting in production of a steel belt,which has a stable profile with stable quality.

The other features of the present invention will be clearly understoodfrom the following Examples.

EXAMPLE 1

Example 1 used a ring-rolling mill, which had a tension roll 3 and areturn roll 4 each of 75 mm in diameter with a couple of work rolls 2 a,2 b of 70 mm in diameter located between the rolls 3 and 4.

A steel strip 1 of 0.35 mm in thickness and 15 mm in width was preparedfrom a metastable austenitic stainless steel, which had a compositionconsisting of 0.086 mass % C, 2.63 mass % Si, 0.31 mass % Mn, 8.25 mass% Ni, 13.73 mass % Cr, 0.175 mass % Cu, 2.24 mass % Mo, 0.064 mass % Nand the balance being Fe except inevitable impurities with a value Md(N)of 74.03. The specified composition allows formation of a dual phasestructure of strain-induced martensite/austenite during aging.

The steel strip 1 was formed to a ring shape with a circumferentiallength of 611 mm by laser-welding its front and tail ends together.

After the steel strip 1 was disposed between the tension roll 3 and thereturn roll 4, it was continuously sent to a gap between the work rolls2 a and 2 b along an endless track with a tension of approximately 5kgf. The steel strip 1 was ring-rolled to a steel belt of 0.20 mm inthickness with a circumferential length of 1070 mm, while controlling arolling load and a tension applied to the steel strip 1 under theconditions that a maximum rolling load, a circumferential speed of thework rolls 2 a, 2 b and a tension of the tension roll 3 were adjusted to3 ton, 2 m/minute and 200 kgf, respectively. Herein, a reduction ratio Rwas 42.9%, and an equivalent strain ε was 0.647.

Three values, i.e. 0° C., 25° C. and 50° C., were preset as a materialtemperature T. A surface temperature of the steel strip 1 was measuredby the noncontact radiation thermometer 9 at a position where the steelstrip 1 was just sent to the gap between the work rolls 2 a and 2 b. Thematerial temperature T of the steel strip 1 was feed-back controlled bychanging a volume of hot air, which was supplied from the generator 8 tothe heating box 10, in response to the measured value.

The rolling conditions are summarized in Table 1.

TABLE 1 Rolling Conditions Reduction Material ratio R(%) Calculated rateα′ Condition temperature (equivalent (vol. %) of strain- No. T (° C.)Md(N) strain ε) X induced martensite I 0 74.03 42.9 80.94 70.07 II 25(0.647) 71.16 60.29 III 50 61.38 50.51 X = −0.3913 T + 0.5650 Md(N) +60.46 ε

A rate of strain-induced martensite in the steel belt manufactured byring-rolling was measured. Results are shown in FIG. 7. It is understoodfrom FIG. 7 that a rate of strain-induced martensite α′ calculatedaccording to the formula of α′=−0.3913T+0.5650Md(N)+60.46ε−10.87 is wellconsistent with the actual measurement value. In fact, strain-inducedmartensite was formed at a rate of 55 vol. % or more under the rollingcondition No. I or II with a value X of 65.78 or more (in other words, acalculated rate of strain-induced martensite α′ being 55 vol. % ormore), but a rate of strain-induced martensite α′ was insufficient underthe rolling condition No. III with a lower value X.

It is noted in FIG. 7 that a rate of strain-induced martensite α′increases as the material temperature T falls down. Cross-sectionalhardness of the steel belt was higher as an increase of strain-inducedmartensite α′. Consequently, the steel belt was more strengthened asfalling of the material temperature T, as shown in FIG. 8. The numeralsallotted to the abscissa of FIG. 8 represent measurement points presetin intervals of 0.25 mm along a circumferential direction of the steelbelt including a welded part, as shown in FIG. 9.

It is confirmed from the above-mentioned results that a rate ofstrain-induced martensite α′ is forecast according to the formula ofα′=−0.3913T+0.5650Md(N)+60.46ε−10.87 and adjusted to 55 vol. % or moreby controlling a material temperature T, an equivalent strain ε and avalue Md(N) so as to satisfy the condition of−0.3913T+0.5650Md(N)+60.46ε≧65.87. As a result, a stainless steel beltexcellent in fatigue property and mechanical strength useful forcontinuously variable transmission is offered.

EXAMPLE 2

A steel strip 1 was formed to a ring shape with a circumferential lengthof 611 mm from the same metastable austenitic stainless steel as Example1, by laser-welding its front and tail ends together. The welded steelstrip was ring-rolled to a steel belt of 0.20 mm in thickness with acircumferential length of 1070 mm under the same conditions as Example 1except for controlling a material temperature T to 10±0.5° C. or 30±0.5°C. at the atmospheric temperature of 10° C. or 30° C., respectively.

For comparison, the same steel strip 1 was ring-rolled at an atmospherictemperature of 10° C. or 30° C. without controlling a materialtemperature T. In this case, the material temperature T was elevated byapproximately 10° C. at a position in the vicinity of an exit of thework rolls 2 a, 2 b, due to generation of processing heat at anyatmospheric temperature of 10° C. or 30° C.

Thickness, width and cross-sectional hardness of each manufactured steelbelt were measured at several points along its circumferentialdirection. Deviations were calculated from the measured values.Calculation results in Table 2 prove that steel belts, which weremanufactured at a controlled material temperature T, had substantiallyuniform thickness, width and cross-sectional hardness with deviationssmaller than halves of steel belts, which were manufactured withoutcontrolling the material temperature T.

TABLE 2 Effects of control of a material temperature T on deviations ofthickness, width and cross-sectional hardness A material temperature T10 ± 0.5° C. 30 ± 0.5° C. Temperature control done none done noneThickness deviation (μm) 2.0 4.4 5.1 6.3 Width deviation (μm) 17 52 1948 Hardness deviation (HV) 4.5 9.8 5.9 14.7

INDUSTRIAL APPLICABILITY

According to the present invention as mentioned above, a rate ofstrain-induced martensite α′, which is formed by ring-rolling ametastable austenitic stainless steel strip, is forecast by the formulaof α′=−0.3913T+0.5650Md(N)+60.46ε−10.87. When a rate of strain-inducedmartensite α′ is adjusted to a value of 55 vol. % or more by controllinga material temperature T, an equivalent strain ε and a value Md(N) so asto satisfy the relationship of −0.3913T+0.5650Md(N)+60.46ε≧65.87, asteel belt manufactured by ring-rolling is bestowed with fatiguestrength similar or superior to a conventional continuously variabletransmission belt made of a 18%-Ni maraging steel. A rolling load isalso alleviated by lowering a material temperature T to a lowestpossible level and a rolling reduction R. Moreover, a rate ofstrain-induced martensite α′ is controlled to a predetermined value witha tolerance of ±2.5 vol. %, by properly confining a variation ΔT of thematerial temperature T during ring-rolling. Consequently, a steel beltexcellent in quality and dimensional accuracy useful for a continuouslyvariable transmission is manufactured from a metastable austeniticstainless steel.

1. A method of manufacturing a continuously variable transmission beltfrom a metastable austenitic stainless steel strip, which comprises thesteps of: forming a metastable austenitic stainless steel strip to aring shape by welding its front and tail ends together; disposing saidring-shaped steel strip between a tension roll and a return roll;continuously sending said ring-shaped steel strip through a gap betweenwork rolls, which are located between said tension roll and said returnroll; and rolling said ring-shaped steel strip under the condition thatthe relationship −0.3913T+0.5650Md(N)+60.46ε≧65.87 is established amonga material temperature T, wherein an equivalent strain ε is defined bythe equation of ε=√{square root over (4(1n(1−R))²/3)} wherein R is thereduction ratio and a value Md(N) is defined by the equation ofMd(N)=580−520C−2Si−16Mn−16Cr−23Ni−300N−10Mo, wherein a variation ΔT ofthe material temperature T is confined within a range of ±6.4° C. duringrolling, and said method is carried out without aging treatment.
 2. Themanufacturing method defined by claim 1, wherein the metastableaustenitic stainless steel strip has the value Md(N) within a range of20–100.
 3. The manufacturing method defined by claim 1, wherein thematerial temperature T is an atmospheric temperature or a surfacetemperature of the steel strip at a position where the steel strip isjust sent to the gap between the work rolls.